1. Field of the Invention
The present invention relates to anti-lock brake systems (ABS) and/or traction control systems (ASR, for anti-slip regulation), more particularly to evacuating and filling an ABS unit, and still more particularly to preventing the system from developing air pockets in a low-pressure accumulator area affecting braking performance.
2. Description of the Related Art
ABS (and ASR) is a part of the standard equipment in medium-class and upper-class vehicles. These systems are becoming increasingly included in lower-category and low-cost cars.
ABS and their variations typically comprise a hydraulic control unit (HCU) that houses hydraulic components of the system. The HCU is connected to the vehicle brake system between its master cylinder and calipers of wheel brakes.
A conventional hydraulic brake system, a one-wheel circuit 10 of which is shown in FIG. 1, comprises a tandem master cylinder (TMC) common for all wheels (shown is an inlet 12 from TMC), a HCU 14, and a wheel caliper (shown in FIG. 1 is an outlet 16 to the wheel caliper). Depicted within dotted lines is a low-pressure portion 18 of the HCU 14. The portion 18 typically contains a low-pressure accumulator 20, an input portion 22 of a return pump 24, and associated lines 26 and 28. A motor 30 drives the return pump 24. Depicted in FIG. 1 are also a return pump outlet valve 32, a noise damper 34, and two electromagnet valves: a pressure buildup normally open (NO) valve 36 and a pressure reduction normally closed (NC) valve 38. The pressure buildup electromagnet NO valve 36 is disposed in a brake line 40 connecting the inlet 12 from TMC to the outlet 16. A check valve 42 is connected in parallel to the NO valve 36. The pressure reduction electromagnet NC valve 38 is placed in a return line 44 that eventually, through the return pump 24 reunites with a main brake line 46.
Prior to filling the system including the circuit 10 with brake fluid, a vacuum is applied at the TMC to remove air from the system. When the vacuum is applied to the HCU, a portion of the unit, namely the low-pressure circuit, is isolated and does not evacuate its air. Any air bubbles still present in the hydraulic medium may bring about such an elasticity of fluid columns formed by the medium that the brake pressures required in wheel brake cylinders are not brought about to the requisite level.
ABS/ASR brake systems have several NC electromagnet valves; the valves of return pumps used in these brake systems are normally closed as well. The problem therefore exists that pockets of air may be present downstream of the NC electromagnet valves, and also downstream of the return pump valves. The air in these pockets may possibly be removed only with difficulty using liquid.
It has been proposed that at least the NC valves be triggered electrically upon evacuation and prior to filling of the system, and thus opened. To do so, special plug connections must be used to supply current to these electromagnet valves. The connections of this kind are expensive and involve operating costs when used, let alone that they are also complicated to manufacture. For many applications, therefore, it is the customer's desire not to energize the NC valves during the evacuation/filling process because of those issues involved.
To avoid the need to electrically energize the valves at the assembly plant, a one-time bleed valve with reset capability for an anti-lock hydraulic control unit has been put forward to address the problem of evacuating the low-pressure circuit. The bleed valve comprises a piston that can be sealingly installed into a bore made in the body of the hydraulic control unit. The bore is sized to create a seal when the piston is pushed into the bore in response to a brake pressure. On one of its ends, the bore is in communication with the primary hydraulic pressure circuit. On another end, it is connected with the low-pressure line. Due to such a structure of the bleed valve, an air communication is established between the primary hydraulic pressure circuit and the low-pressure line when the piston sits loosely in the bore. By virtue of that communication, air can be evacuated from the low-pressure brake circuit when it is exposed to a vacuum applied to the primary hydraulic pressure circuit.
However, all the advantages of such a solution notwithstanding, the placing of the bleed valve into the hydraulic system still may not fully prevent a vacuum from developing over time in the low pressure accumulating area after several normal brake pedal depressions. Specifically, the vacuum can be created upon brake pedal release. The accumulated vacuum may lead to air gaining access into the system and thus to a "soft pedal". Also, when the HCU subsequently goes into an ABS mode, the trapped air can be pumped into the brake circuit causing reduced brake performance.
Accordingly, a need as yet exists in the art to provide a reliable means for evacuating the low-pressure circuit.